This invention relates to electronic device fabrication processes. More specifically, the invention relates to resolving the fluorine loading effect in the reaction chamber of a HDP CVD apparatus used for forming dielectric layers in high aspect ratio, narrow width recessed features.
It is often necessary in semiconductor processing to fill high aspect ratio gaps with insulating material. This is the case for shallow trench isolation, inter-metal dielectric layers, passivation layers, etc. As device geometries shrink and thermal budgets are reduced, void-free filling of high aspect ratio spaces (e.g., AR>3:1) becomes increasingly difficult due to limitations of existing deposition processes.
Most deposition methods deposit more material on the upper region than on the lower region of a sidewall and/or form top-hats at the entry of the gap. As a result the top part of a high aspect ratio structure sometimes closes prematurely leaving voids within the gap's lower portions. This problem is exacerbated in small features. Furthermore, as aspect ratios increase, the shape of the gap itself can contribute to the problem. High aspect ratio gaps often exhibit reentrant features, which make gap filling even more difficult. One such problematic reentrant feature is a narrowing at the top of the gap. The etched sidewalls slope inward near the top of the gap. For a given aspect ratio feature, this increases the ratio of gap volume to gap access area seen by the precursor species during deposition. Voids and seams formation is more likely under these conditions. If the top of the gap prematurely closes off, a chemical etch is required to re-open the gap before more film can be deposited in the gap.
High-density plasma chemical vapor deposition (HDP CVD) is a directional (bottom-up) CVD process that is used for high aspect ratio gap-fill. The method deposits more material at the bottom of a high aspect ratio structure than on its sidewalls. It accomplishes this by directing charged dielectric precursor species downward, to the bottom of the gap. Thus, HDP CVD is not an entirely diffusion-based (isotropic) process.
Nevertheless, some overhang or top-hat formation still results at the entry region of the gap to be filled. This results from the non-directional deposition reactions of neutral species in the plasma reactor and from sputtering/redeposition processes. The directional aspect of the deposition process produces some high momentum charged species that sputter away bottom fill. The sputtered material tends to redeposit on the sidewalls. Limitations due to overhang formation become ever more severe as the width of the gap to be filled decreases and the aspect ratio increases.
Conventional HDP CVD processes rely on plasma etch steps to remove sidewall deposits and top-hats. Typically a fluorine species, such as NF3, is used between dielectric film deposition steps to etch the film. After a layer of dielectric partially fills gaps on a substrate, the fluorine-containing plasma etches the layer to remove top-hats and open the gap for further deposition. However, these etch steps are time-consuming and expensive.
The current gap fill process uses multiple sequences of deposition and etch and this process methodology leaves behind residual fluorine which remains incorporated in trace amounts in the chamber, primarily the ceramic dome. These trace amounts accumulate with time and cause a detrimental fluorine loading effect resulting in a shift in the stack film thickness on the wafer in the process chamber as wafers are processed. The loading effect is mainly caused by fluorine attacking the chamber environment and increases the batch thickness drift as more wafers are produced.
To improve fabrication of advanced technology devices, the art requires better dielectric deposition and etch processes that can fill high aspect ratio features of narrow width and reduce sidewall and top-hat formation without the detrimental fluorine loading effects on drift in the thickness of the deposition as the fabrication processes proceed.